As an antitheft system of this type, there is disclosed, for example, in JP 2000-73411 A a technology that detects, for example, by a global positioning system, i.e., GPS the position of a hydraulic excavator as a self-propelling movable object under control, transmits by wireless communication the thus-detected position of the hydraulic excavator to a control server located at a remote place, and ascertains by the control server whether or not the position of the hydraulic excavator is within a predetermined normal work area.
According to this conventional technology, when the position of the hydraulic excavator as detected by GPS has been determined to depart from the normal work area, an engine stop signal is transmitted from the control server to a control system mounted on the hydraulic excavator to stop an operation of the hydraulic excavator.
As described above, the conventional technology makes it possible to promptly infer the theft of the hydraulic excavator and to disable work such as digging or running by the hydraulic excavator and, because the hydraulic excavator becomes no longer possible of self-propelling, also makes it difficult to load the hydraulic excavator on a vehicle for carrying it away, for example, a trailer. Accordingly, the conventional technology can scare away a potential thief and practically, can make it difficult to steal, thereby serving as an effective technology for the prevention of a theft.
Among self-propelling movable objects, construction machines such as hydraulic excavators, in particular, are often stolen at night after finishing work. While being transported on a trailer or the like, the engine of the hydraulic excavator is in a stopped state. There is, accordingly, a need for the development of a countermeasure for a theft while the engine is stopped. In the above-described conventional technology, no reference is specifically made as to the time during which the engine is stopped. When constructed, for example, to continuously feed power to a control system, which performs positional detections and transmissions/receptions to/from a control server, always including the time during which the engine is stopped, the voltage of a power supply, i.e., a battery mounted on the hydraulic excavator, however, drops (undergoes a battery drainage) in a short time so that frequent recharging is required. When constructed, as in the conventional technology, to transmit the position information on the hydraulic excavator to the side of the control server and to determine on the side of the control server whether or not the hydraulic excavator has departed from a normal work area, on the other hand, the number of communications between the hydraulic excavator and the control server becomes great, resulting in a substantial communication cost. Therefore, a problem also remains unsolved in this respect.
The present invention has been completed in view of the above-described problems of the conventional technology. A first object of the present invention is, therefore, to provide an antitheft system which can ascertain the position of a movable object even during stopping of an engine while reducing the consumption of power by a control system during the stopping of the engine and avoiding a battery drainage. Further, a second object of the present invention is to provide an antitheft system which can keep the communication cost low by minimizing the number of communications with a control server.